1. Field of the Invention
This invention relates to pistons for use in master cylinders which are used, for instance, for brakes, and more particularly to a piston of this type which is made of resin.
2. Description of the Prior Art
The piston for a master cylinder which is made of aluminum or iron is well known in the art, and it is generally high in manufacturing cost, being formed by machining, hardening and so forth. In order to overcome this difficulty, a piston made of resin has been proposed in the art. However, the piston made of resin cannot satisfy the requirement that the piston for a master cylinder should be sufficiently high in compressive strength. In the case where the piston is made of resin, voids (bubbles) may be formed inside it, because the inner and outer surfaces of the piston made of resin are different in the speed of coagulation from each other depending on the wall thickness of the latter. That is, the piston made of resin may be low in compressive strength. In order to eliminate this difficulty, a master cylinder with a piston made of resin has been proposed by Japanese Utility Model Unexamined Publication No. Hei 4-43558. FIG. 3 is a sectional view of the conventional piston made of resin thus proposed. The master cylinder comprises: a master cylinder body; a piston made of resin (hereinafter referred to as "a resin piston") which is slidably supported in the master cylinder body; and a metal member an axis of which is in alignment with the axis of the resin piston, the metal member being inserted into the resin piston to reinforce the latter. In the master cylinder, the metal member includes: an exposing portion which is exposed at only one end of the resin piston as viewed in the direction of axis of the latter; and a burying portion with a groove which is buried in the resin piston. The resin piston and the metal member are provided as one unit by insert injection molding.
The above-described piston for the master cylinder disclosed by the aforementioned Japanese Utility Model Publication No. Hei 4-43558 suffers from the following difficulties: In order to make the resin piston high in compressive strength, the piston is formed by insert injection molding with the metal member set inside it. In the resin piston, its portion around the metal member is larger in the thickness of resin than the remaining portions; that is, the piston is not uniform in the thickness of resin. The relatively thicker portion of the piston, and the relatively thinner portion of the latter are different from each other in the speed of coagulation, and voids are liable to be formed in the relatively thicker portion of the piston. This fact makes it difficult to make the relatively thicker portion of the piston high in compressive strength.
In the insert injection molding operation, for instance the setting of the metal member takes time. Hence, the insert injection molding method is not suitable for the formation of the resin piston, being low in work efficiency.
Furthermore, the conventional piston has an undercut to fix a seal ring. Hence, in manufacturing the piston, a parting line is formed on its portion which is brought into contact with the inner cylindrical surface of the cylinder. Therefore, the manufacture of the piston must have a step of removing the parting line. This is one of the factors which increases the number of manufacturing steps, and accordingly the manufacturing cost.